These days, unmanned aerial vehicles are used in many fields of technology as a result of flexible employability, be it to reach terrain sections that are difficult to access, for example when fighting fires or in disaster zones, or to carry out an image-supported examination of large objects. In order to capture terrain information, such instruments can be equipped with sensors, e.g. with cameras, and relatively large terrain sections can be recorded contiguously therewith from the air. Furthermore, corresponding drones can be employed for military purposes, e.g. for monitoring, target acquisition, as combat unit or transport means.
In principle, an unmanned aerial vehicle can be controlled or moved manually by means of a remote control by a user or in a completely autonomous or semiautonomous fashion, usually on the basis of GNSS position information.
In general, it is possible to modify four from six degrees of freedom when moving the aerial vehicle, e.g. a helicopter-like aerial vehicle, i.e. the aerial vehicle can be moved forward and backward, left and right and up and down. Moreover, the alignment of the aerial vehicle can be modified by a rotation about the vertical axis. The remaining two degrees of freedom are fixed by the substantially horizontal position of the aerial vehicle.
Precise positioning in a predetermined position or precise movement, e.g. along a predefined axis or flight route, was found to be difficult for a user in the case of manual control. Particularly if the aerial vehicle is exposed to external influences, such as e.g. wind, and the deviations created thereby have to be compensated for with quick reactions, a required accuracy can often not be maintained in the case of such a manual control.
Furthermore, the field of application for an autonomous GNSS-based control is limited to locations at which a sufficient number of satellite signals can be received for determining the position. Hence, in general, a use in e.g. closed rooms or tunnels is not possible. The use in heavily built-up areas can also be difficult if buildings shield GNSS signals.
In order to control an aerial vehicle in such a built-up area, EP 1 926 007 proposes a first flyby over the relevant area, during which images are taken and GPS information is stored with each image. The images are subsequently combined to form an overview image with GPS position information. In order to navigate the aerial vehicle, the images which are recorded at a lower altitude than the ones recorded in advance can now be compared to the overview image and a respective position of the aerial vehicle can be derived on the basis of the stored GPS information. Disadvantages in this procedure can emerge if the first overview image does not comprise all areas of the buildings and the spaces between the buildings and it proves impossible to find correspondence in the case of an image comparison. Positional determination can also be impaired by changes in the surroundings captured at first, for example by movement of vehicles depicted in the image or if light conditions change. Furthermore, this method is limited by the resolution of the camera capturing the surroundings.
EP 1 898 181 discloses a further system and method for controlling an unmanned aerial vehicle, wherein GPS signals, measurement data from inertia sensors and images captured by a camera are used for determining or estimating a position of the aerial vehicle. The captured signals and data can be fed to a computer unit and the position can be determined therefrom. By using the camera, carrying out this determination of the position can supply more reliable results compared to systems without a camera and enable an increased accuracy. However, this method is also limited by the resolution of the camera or can possibly only be carried out to restricted extent as a result of changes in the captured surroundings.
In the case of an autonomous control, the route can furthermore be prescribed to the aerial vehicle in the form of a trajectory, for example it can be defined by several waypoint positions. EP 2 177 966 describes a navigation method for a aerial vehicle on the basis of a predetermined flight route, wherein, for the purposes of controlling the aerial vehicle, pictures of the flight surroundings can be taken by a camera and the flight route can be adapted on the basis thereof. In order to control the aerial vehicle on the flight route, specific intended positions or waypoint positions can be compared to a current actual position of the aerial vehicle, which can, for example, be determined by the GNSS signals. Control signals for the movement of the aerial vehicle can thus be determined from the differences in position and, as a result thereof, a deviation of the actual position with respect to the target position can successively be reduced.
What is common to the aforementioned methods or systems is that the position of the aerial vehicle, in particular the vertical position, can only be determined to an accuracy of up to 2-5 cm by means of GNSS sensors. This uncertainty subsequently has a great limitation on the accuracy when determining the position of the aerial vehicle and on the accuracy when controlling the aerial vehicle.